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MATTER AND FORCE. 85

through this state of motion, to its final crystalline re-
pose.

I can show you something similar. Over a piece of
perfectly clean glass I pour a little water in which a crystal
has been dissolved. A film of the solution clings to the
glass, and this film will now be caused to crystallize before
your eyes. By means of a microscope and a lamp, an image
of the plate of glass is thrown upon the screen. The beam
of the lamp, besides illuminating the glass, also heats it;
evaporation sets in, and, at a certain moment, when the
solution has become supersaturated, splendid branches of
crystals shoot out over the screen. A dozen square feet of
surface are now covered by those beautiful forms. With
another solution we obtain crystalline spears, feathered
right and left by other spears. From distant nuclei in
the middle of the field of view the spears shoot with magical
rapidity in all directions. The film of water on a window-
pane on a frosty morning exhibits effects quite as wonderful
as these. Latent in this formless solution, latent in every
drop of water, lies this marvellous structural power, which
only requires the Withdrawal of opposing forces to bring it
into action.

Our next experiment on crystallization you will probably
consider more startling even than these. The clear liquid
now held up before you is a solution of nitrate of silver—a
compound of silver and nitric acid. When an electric cur-
rent is sent through this liquid the silver is severed from
the acid, as the hydrogen was separated from the oxygen
in a former experiment; and I would ask you to observe
how the metal behaves when its molecules are thus succes-
sively set free. The image of the cell, and of the two wires
which dip into the liquid of the cell, are now clearly shown
upon the screen. Let us close the circuit, and send the
current through the liquid. From one of the wires a beau—
tiful silver tree commences immediately to sprout. Branches

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